Susceptor for deposition apparatus

ABSTRACT

An apparatus for depositing a material on a wafer includes a susceptor plate mounted in a deposition chamber. The chamber has a gas inlet and a gas exhaust. Means are provided for heating the susceptor plate. The susceptor plate has a plurality of support posts projecting from its top surface. The support posts are arranged to support a wafer thereon with the back surface of the wafer being spaced from the surface of the susceptor plate. The support posts are of a length so that the wafer is spaced from the susceptor plate a distance sufficient to allow deposition gas to flow and/or diffuse between the wafer and the susceptor plate, but still allow heat transfer from the susceptor plate to the wafer mainly by conduction. The susceptor plate is also provided with means, such as retaining pins or a recess, to prevent lateral movement of a wafer seated on the support posts.

This is a continuation of application Ser. No. 08/752,742, filed Nov.14, 1996, now U.S. Pat. No. 5,645,646, which is a continuation ofapplication Ser. No. 08/536,650, filed Sep. 29, 1995 abandoned, which isa continuation in part of application Ser. No. 08/202,142, filed Feb.25, 1994 abandoned.

FIELD OF THE INVENTION

The present invention is directed to a susceptor for supporting a waferin a deposition apparatus for depositing a layer of a material on thesurface of the wafer, and, more particularly, to a susceptor whichallows deposition on both surfaces of the wafer.

BACKGROUND OF THE INVENTION

One type of deposition apparatus used in the semiconductor industry is asingle wafer chamber apparatus in which one wafer at a time is placed inthe deposition chamber and a layer of a material is deposited on thewafer. Referring to FIG. 1, there is shown one form of a typical singlewafer deposition apparatus 10. Deposition apparatus 10 comprises anenclosed chamber 12 formed by an upper dome 14, a lower dome 16 and aside wall 18 between the upper and lower domes 14 and 16. A flatsusceptor 20 is mounted in and extends across the chamber 12. Thesusceptor is generally mounted on a shaft 22 which provides for rotationof the susceptor 20. A pre-heat ring 21 is around the periphery of thesusceptor 20 and extends between the susceptor 20 and the side wall 18.A deposition gas inlet port 24 extends through the side wall 18 of thechamber 12. An exhaust port 26 also extends through the side wall 18 ofthe chamber 12. An exhaust port 26 also extends through the side wall 18substantially diagonally opposite the inlet port 24. Heating means, suchas lamps 28, are mounted around the chamber 12 and direct their lightthrough the upper and lower domes 14 and 16 onto the susceptor 20 andpre-heat ring 21 to heat the susceptor 20 and the pre-heat ring 21. Adoor, not shown, is also provided in the side wall 18 through whichwafers can be inserted into and removed from the chamber 12.

To deposit a layer of a material, such as silicon, on the surface of awafer 30, the wafer 30 is placed on the susceptor 20 and a flow of adeposition gas is provided across the chamber 12 from the inlet port 24to the exhaust port 26. To deposit silicon, the gas used contains asilicon containing material, such as silane, and an inert carrier gas,such as hydrogen. The susceptor 20 and wafer 30 are heated by the lamps28. The heated susceptor 20, pre-heat ring 21 and wafer 30 heat thedeposition gas passing over their surfaces causing the gas to react anddeposit a layer of the material, silicon, on the surface of the wafer30.

The heat from the lamps 28 is absorbed by the susceptor 20 and isnormally transferred to the wafer 30 by conduction across a thininterface between the wafer 30 and the susceptor 20. The wafer 30 andsusceptor 20 are in fairly intimate contact with the wafer 30 beingmounted on the susceptor 20. However, as shown in FIG. 2, the surface ofthe susceptor 20 is not perfectly smooth, but has some roughness.Because of the surface roughness of the susceptor 20, the wafer 30actually only contacts the surface of the susceptor 20 at a few points.The flatness of the wafer 30 and the susceptor 20 are within a fewthousandths of an inch so that the heat is readily conductive throughthe gas molecules that occupy the space between the wafer 30 and thesusceptor 20. As shown in FIG. 3, in some cases, the surface of thesusceptor 20 is curved or recessed away from the wafer 30 (typically 2to 10 thousands of an inch) to avoid hot spots resulting from the pointcontact of the wafer and susceptor. However, in either case, the primaryheat transfer between the wafer 30 and susceptor 20 is conduction acrossthe gas film between the two surfaces. Radiation heat transfer doesoccur, but is of secondary importance because of the close spacing ofthe wafer 30 to the susceptor 20.

During the process of depositing a layer of a material, such as silicon,on the surface of the wafer 30, molecules containing the material,typically silane for silicon, diffuse to the back surface of the wafer30. This results from the fact that the molecules are driven by apartial pressure difference since there are initially no such moleculesbetween the wafer 30 and the susceptor 20. These atoms thermallydecompose and deposit silicon on the back surface of the wafer 30 andthe corresponding surface of the susceptor 20. However, this depositionis typically confined to a few millimeters from the edge of the wafer30. For the manufacture of certain types of semiconductor devices, suchas integrated circuits, it is sometimes desirable to deposit silicon onthe entire back surface of the wafer 30 as well as on the front surfaceof the wafer 30. Although uniformity of the layer deposited on the backsurface of the wafer 30 is not an issue, it is important to havecomplete coverage. Also, for time saving, it would be desirable to beable to coat the back surface of the wafer 30 at the same time that thecoating is being applied to the front surface of the wafer 30.

As one example of the need for complete coverage of a coated wafer in acold wall CVD process where the wafer is being prepared for certainapplications, FIG. 7 demonstrates a wafer 70 in which incompletecoverage of the underside has taken place. As shown, a polysilicon layer71 is deposited on a silicon wafer substrate 72. In most applicationsthe wafer substrate will generally have a thin coating of silicondioxide (SiO₂) 73 which acts as an interface layer for the overcoatedpolysilicon layer 71.

When a polysilicon coating 71 is applied by means of a cold wall CVDapparatus as shown in FIG. 1, the coverage on the wafer is incomplete,generally coating the outer periphery of the underside of the wafer to adistance of 1/4 inches to 3/4 inches from the wafer's edge of the wafer.This leaves a significant portion 76 of the underside oxide interfacecoating exposed which increases the risks of a defective wafer incertain applications. For example, certain wafer uses require ahydrofluoric acid (HF) dip as a step after the chemical vapor depositionof the polysilicon. In such a step the HF will etch the exposedunderside portion 76 of the silicon dioxide layer 73. This erosion willcause a gap 75 between the polysilicon 71 and the underside oxide. Suchloose ends of the polysilicon as shown in FIG. 7 will enevitablygenerate flakes of loose polysilicon particles. These particulate flakesbecome contaminants to the HF bath and the wafer itself becomes a sourceof potential contamination in any subsequent fabrication processing ofthe wafer. This example amply demonstrates the need for a device andprocess for securing total coverage of a wafer substrate in a coldwall/heated susceptor application.

SUMMARY OF THE INVENTION

One aspect of the present invention is directed to a susceptor for anapparatus for depositing a layer of a material on the surface of awafer. The susceptor comprises a plate having a surface. A plurality ofsupport posts project from the surface of the plate. The support postsare arranged in spaced relation in a pattern which permits a wafer to beseated on the support posts with the wafer being spaced from the surfaceof the plate.

Another aspect of the present invention is directed to an apparatus fordepositing a layer of a material on a wafer which comprises a depositionchamber, a circular susceptor plate in the chamber and mounted on ashaft for rotation, a gas inlet into the chamber, an exhaust from thechamber and means for heating the susceptor plate. The susceptor platehas a plurality of support posts projecting from a top surface thereof.The support posts are arranged in spaced relation in a pattern whichpermits a wafer to be seated on the support posts with the wafer beingspaced from the surface of the susceptor plate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a prior art single waferdeposition apparatus;

FIG. 2 is an enlarge sectional view of a portion of one form of asusceptor used in the apparatus shown in FIG. 1;

FIG. 3 is an enlarge sectional view of a portion of another form of asusceptor used in the apparatus shown in FIG. 1;

FIG. 4 is a sectional view of a portion one form of the susceptor inaccordance with the present invention;

FIG. 5 is a top view of the susceptor shown in FIG. 4;

FIG. 6 is a sectional view of a portion of another form of a susceptorin accordance with the present invention; and

FIG. 7 is a sectional view of a wafer processed in accordance with theprior art.

DETAILED DESCRIPTION

Referring now to FIG. 4, there is shown a sectional view of one form ofa susceptor 32 in accordance with the present invention. Susceptor 32 isa circular plate having a relatively flat top surface 34, and a shaft(not shown) secured to the center of its bottom surface. Projectingupwardly from the top surface 34 are plurality of support posts 36. Thesupport posts 36 are in spaced relation around a circle which is of adiameter slightly less than the diameter of the wafer 30 to be treatedon the susceptor 32. There should be at least three support posts 36,and as shown in FIG. 5, there are preferably six support posts 36 on thesusceptor 32. Thus, the support posts 36 are adapted to support a wafer30 thereon with the back surface 31 of the wafer 30 being slightlyspaced from the top surface 34 of the susceptor 32. The support posts 36are of a length to space the back surface 31 of the wafer 30 from thetop surface 34 of the susceptor 32 a distance sufficient to allowmolecules of the deposition gas to diffuse completely across the backsurface of the wafer 30. However, the spacing mush be close enough toprovide for a significant portion of the heat transfer from thesusceptor 32 to the wafer 30 to be conduction; that is, the spacing mustminimally be of a value to retain heat conduction coupling between theheated susceptor 30 and the wafer substrate 32 to be coated. However,since the wafer 30 is spaced from the surface 34 of the susceptor 32,radiation heat transfer becomes much more important than when the waferis directly on the surface of the susceptor. While the lengths of thesupport pins 36 can be of any value that achieves the heat couplingbetween the susceptor 32 and the substrate 30 to accomplish the fullcoverage coating of the wafer substrate, typical lengths range from 0.5millimeters to 10 millimeters. Preferred wafer coating results have beenfound with pin lengths of 1 millimeter to 6 millimeters.

The diameter of the support pins 36 should be sufficient to provide goodsupport for the wafer 30, but not large enough to cover a significantarea of the back surface 31 of the wafer 30. A typical diameter for thesupport pins 36 is from one to three millimeters.

Also projecting from the top surface 34 of the susceptor 32 are aplurality of retaining pins 38. The retaining pins 38 are arranged inspaced relation around a circle which is of a diameter slightly largerthan the diameter of the wafer 30 to be treated on the susceptor 32.Although three of the retaining pins 38 can be used, as shown in FIG. 5,six of the retaining pins 38 are preferred. The retaining pins 38 are ofa length longer than the length of the support pins 36, preferably by adistance at least equal to the thickness of the wafer 30. Typically, theretaining pins 38 are of a length at least one-half millimeter longerthan the length of the support pins 36.

The susceptor 32 of the present invention is used in a heatedsusceptor/cold wall deposition apparatus 10 such as shown in FIG. 1 andreplaces the susceptor 20 shown in FIG. 1. A wafer 30 is placed on thesusceptor 32 and is seated on the support pins 36 within the retainingpins 38. The retaining pins 38 prevent lateral movement of the wafer 30as the susceptor 32 rotates during the deposition operation. Adeposition gas is fed into the chamber 12, and the susceptor 32 isheated by the lamps 28. The susceptor 32 heats the wafer 30 mainly beconduction, but partially by radiation. The deposition gas not onlyflows across the top surface of the wafer 30, but also flows and/ordiffuses between the back surface 31 of the wafer 30 and the top surface34 of the susceptor 32. Since the back surface 31 of the wafer 30 isspaced from the top surface 34 of the susceptor 32 by the support posts36, some of the deposition gas flows and/or diffuses completely acrossthe back surface 31 of the wafer 30. The heated wafer 30 heats thedeposition gas which contacts its surfaces causing the deposition gas toreact and deposit a layer of the material over both surfaces of thewafer 30. Thus, the susceptor 32 of the present invention provides forcoating the back surface 31 of the wafer 30 at the same time that thetop surface is coated, but still achieves heating of the wafer 30 mainlyby conduction from the susceptor 32.

Referring now to FIG. 6, there is shown another form of a susceptor 40in accordance with the present invention. Susceptor 40 comprises acircular plate having a top surface 42 and a circular recess 43 in thetop surface 42. A plurality of support posts 44 project upwardly fromthe bottom surface 46 of the recess 44. The support posts 44, like thesupport posts 36 of the susceptor 32, are arranged in spaced relationaround a circle which is of a diameter slightly less than the diameterof the wafer 30 to be supported thereon. The susceptor 40 can have thesame number of support posts 44 as the susceptor 32 shown in FIGS. 4 and5. The support posts 44 are of a length to support the wafer 30 spacedfrom the bottom surface 46 of the recess 43, but still allow heattransfer from the susceptor 40 to the wafer 30 mainly be conduction. Thesupport posts 44 typically have a length of one-half to two millimetersand have a diameter typically about one to three millimeters. The recess44 is of a diameter slightly greater than the diameter of the wafer 30to be treated on the susceptor 40. Also, the depth of the recess 43 isgreater than the length of the support posts 44, typically by aboutone-half millimeter or more.

The susceptor 40 is also used in the deposition apparatus 10 shown inFIG. 1 in place of the susceptor 20. In the use of the susceptor 40, awafer 30 is placed in the recess 43 in the susceptor 40 with the backsurface 31 of the wafer 30 being seated on the support posts 44. A flowof a deposition gas is provided in the chamber 12 and the susceptor 40and wafer 30 are heated by means of the lamps 28. Additionally, someheat is transferred from the susceptor 40 to the wafer 30. This heatsthe deposition gas which flows over the top surface of the wafer 30 sothat the gas reacts to deposit a layer of a material on the top surfaceof the wafer 30. At the same time, some of the deposition gas flowsand/or diffuses around the edge of the wafer 30 and between the backsurface 31 of the wafer 30 and the bottom surface 46 of the recess 43.The gas which is between the back surface 31 of the wafer 30 and thebottom surface 46 of the recess 43 is heated and reacts to deposit alayer of the material on the back surface 31 of the wafer 30. Thus, alayer of the material is simultaneously deposited on both the top andbottom surfaces of the wafer 30.

Thus, there is provided by the present invention a susceptor for adeposition apparatus which supports a wafer slightly spaced from thesurface of the susceptor. This allows the deposition gas to pass bothover the top surface of the wafer and between the bottom surface of thewafer and the susceptor. Thus, when the deposition gas is heated by theheated wafer, the deposition gas reacts to deposit a layer of thematerial simultaneously on both the top and bottom surfaces of thewafer. Although the wafer is supported spaced from the surface of thesusceptor, the spacing is small enough that the heat transfer betweenthe susceptor and the wafer is mainly by conduction and only a smallpart by radiation. Thus, the wafer is still readily and quickly heatedby the susceptor. Although the support pins and retaining pins are shownas being arranged in a circle to support a circular wafer, the pins canbe arranged in any shaped closed figure, such as square, rectangular,triangular, etc., to support a wafer of any shape.

What is claimed is:
 1. A apparatus for depositing a layer of a materialon a surface of a wafer comprising:a deposition chamber having asidewall; a gas inlet formed through said sidewall; a gas outlet formedthrough said sidewall opposite said gas inlet; a plate having acontinuous surface positioned in said deposition chamber; and aplurality of support posts coupled to and projecting from the continuoussurface of the plate, said support posts being arranged in spacedrelation in a pattern which permits a wafer to be seated on the supportposts with the wafer being spaced from the surface of the plate saidsupport posts being of a length to space the wafer from the surface ofthe plate a distance sufficient to allow gas to flow and/or diffusebetween the surface of the plate and substantially the entire backsurface of the wafer, but still allow heat transfer from the plate tothe wafer mainly by conduction.
 2. The susceptor of claim 1 in which thesusceptor posts are arranged along a circle whose diameter is slightlyless than the diameter of the wafer to be seated hereon.
 3. Thesusceptor of claim 2 in which there are at least three support postsprojecting from the surface of the plate.
 4. The susceptor of claim 3 inwhich there are six support posts projecting from the surface of theplate.
 5. The susceptor of claim 2 in which the support posts are of alength of between one-half to ten millimeters.
 6. The susceptor of claim5 in which the support posts are of diameter of about three millimeters.7. The susceptor of claim 2 in which the support posts are of a lengthof between one to six millimeters.
 8. The susceptor of claim 2 furthercomprising a plurality of retaining pins projecting from the surface ofthe plate, said retaining pins being longer than the support posts andbeing positioned so that a wafer mounted on the support posts is withinthe retaining pins.
 9. The susceptor of a claim 8 in which the retainingpins are spaced around a circle whose diameter is slightly greater thanthe diameter of the wafer to be seated on the support posts.
 10. Thesusceptor of claim 9 in which the retaining pins are of a length atleast one-half millimeter longer than the length of the support pins.11. The susceptor of claim 2 in which the plate has a top surface and arecess in the top surface, said recess having a bottom surface and thesupport posts project from the bottom surface of the recess.
 12. Thesusceptor of claim 11 in which the recess has a diameter which isslightly greater than the diameter of the wafer to be seated on thesupport posts, and has a depth greater than the length of the supportposts.
 13. An apparatus for depositing a layer of a material on a wafercomprising:a deposition chamber having a sidewall; a circular susceptorplate having a continuous surface in the chamber and mounted on a shaftfor rotation, said susceptor plate having a surface at the top thereofand a plurality of support posts projecting from said surface, saidsupport posts being arranged in spaced relation in a pattern whichpermits a wafer to be seated on the support posts with the wafer beingspaced from the surface of the susceptor plate; and wherein the supportposts on the susceptor plate are of a length to space the wafer from thesurface of the susceptor plate a distance sufficient to allow adeposition gas to flow and/or diffuse between the surface of thesusceptor plate and substantially the entire back surface of the waferbut still allow heat transfer form the susceptor plate to the wafermainly by conduction; a gas inlet into the chamber through saidsidewall; a gas outlet from the chamber said gas outlet formed throughsaid sidewall and opposite said gas inlet; and means for heating thesusceptor plate.
 14. The apparatus of claim 13 in which the supportposts are arranged along a circle whose diameter is slightly less thanthe diameter of the wafer.
 15. The apparatus of claim 14 in which thereare at least three support posts projecting from the surface of thesusceptor plate.
 16. The apparatus of claim 13 in which the supportposts are of length of between one-half to 10 millimeters and of adiameter of about three millimeters.
 17. The apparatus of claim 13further comprising a plurality of retaining pins projecting from thesurface of the susceptor plate, said retaining pins being longer thanthe support posts and being positioned so that a wafer seated on thesupport post is within the retaining pins.
 18. The apparatus of claim 17in which the retaining pins are spaced around a circle whose diameter isslightly greater than the diameter of the wafer to be seated on thesupport posts.
 19. The apparatus of claim 18 in which the retaining pinsare of a length at least one-half millimeter longer than the length ofthe support posts.
 20. The susceptor of claim 13 in which the susceptorplate has a recess in its top surface with the recess having a bottomsurface, and the support posts project from the bottom surface of therecess.
 21. The susceptor of claim 20 in which the recess has a diameterwhich is slightly greater than the diameter of the wafer to be seated onthe support posts and a depth which is slightly greater than the lengthof the support posts.